Field
The presently disclosed subject matter relates to a piezoelectric device, its manufacturing method and an optical deflector using the piezoelectric device.
Description of the Related Art
Recently, piezoelectric devices such as piezoelectric actuators and piezoelectric sensors used in optical scanners have been micro electro mechanical system (MEMS) devices manufactured by semiconductor manufacturing technology and micro machine technology.
In the above-described MEMS device, a piezoelectric layer is formed on a silicon substrate (wafer) by a dry deposition process such as a sputtering process, an ion plating process, a metal organic chemical vapor deposition (MOCVD) process, a pulse laser deposition (PLD) process and a molecular beam epitaxial (MBE) process, or a wet deposition process such as a chemical solution deposition (CSD) process.
In FIG. 1, which is a cross-sectional view for explaining a first prior art method for manufacturing a piezoelectric device by a dry deposition process, one piezoelectric layer such as one lead zirconate titanate PbZrxTi1-xO3 (PZT) layer 104 is continuously grown on a silicon substrate 101, to realize a columnar crystalline structure of PZT whose piezoelectric constant (−d31) becomes large. Therefore, even if the PZT layer 104 is much thicker, the PZT layer 104 would not be peeled off from the substrate 101.
In FIG. 1, note that reference numeral 102 designates an insulating layer, 103 designates a lower electrode layer, and 105 designates an upper electrode layer.
In the columnar crystalline structure, however, Pb components are diffused and segregated into grain boundaries 104a thereof indicated by X, so that electric properties along the thickness direction would deteriorate. Particularly, if segregated Pb components are combined with each other to generate leakage current paths, leakage currents would flow through the segregated Pb components so that the reliability would deteriorate.
Also, the thickness of the PZT layer 104 has recently been increased from about 1 to 2 μm to about 4 to 5 μm to increase the output power of the piezoelectric device. In this case, the roughness at a top surface of the PZT layer 104 would be increased, so that the peak-to-valley (PV) value of the top surface is about 200 nm. Therefore, electric fields would be concentrated on the rough surface of the PZT layer 104 to dielectrically break it down. Further, when the piezoelectric device is driven to grow cracks in the PZT layer 104, water in the air would be immersed into the cracks, which also would increase the leakage currents through the segregated Pb components of the PZT layer 104.
The larger the columnar structure of the PZT layer 104, the larger the roughness of the top surface of the PZT layer 104. Also, the larger the roughness of the top surface of the PZT layer 104, the worse the piezoelectric property of the PZT layer 104.
In order to reduce the leakage currents and the roughness of the top surface of the PZT layer, in a second prior art method for manufacturing a piezoelectric device by a dry deposition process, multiple PZT growing operations, where the substrate temperature is higher than the Curie temperature of PZT, are carried out, and one no-PZT-growing operation, where the substrate temperature is lower than the Curie temperature of PZT, is carried out between the PZT growing operations (see: JP2007-116006). In the no-PZT-growing operation, atoms in the grown. PZT layers would be rearranged and oxygen would be introduced into oxygen deficient portions of the grown PZT layers, thus realizing a columnar crystalline PZT whose thickness is relatively large. For example, a total time period for the multiple PZT growing operations is about 4 hours, while a total time period for the no-PZT-growing operation is about 1 hour.
In the above-described second prior art method, however, since the time period for the no-PZT-growing operation is large, the throughput of piezoelectric devices is low. Also, since Pb components would be evaporated from the grown PZT layers during the no-PZT-growing operation, the piezoelectric property would deteriorate.
In a third prior art method for manufacturing a piezoelectric device by a dry deposition process, a relatively-low dielectric piezoelectric layer made of PbTiO3 (PT) or the like is provided between the PZT layer and its adjacent electrode layers, thus preventing Pb components of PZT from being diffused into the electrode layers. Therefore, leakage currents therethrough can be reduced.
In the above-described third prior art method, however, since the intensity of an internal electric field of the PZT layer is decreased, the displacement amount of the piezoelectric device would be decreased.
On the other hand, in a prior art method for manufacturing a piezoelectric device by a wet deposition process such as a CSD process, multiple processes each including one spin-coating operation, one drying operation and one calcining operation are carried out, so that leakage current paths in the thickness direction can be reduced and the breakdown voltage can be increased.
In the above-described method by a wet deposition process, however, the piezoelectric property generally would not be so high as compared with that of the dry process.